EPR study of photoinduced reduction of nitroso compounds in titanium dioxide suspensions

The radical intermediates produced upon UV irradiation of deoxygenated alcoholic titanium dioxide suspensions of nitrosobenzene, nitrobenzene, 2-nitrosotoluene, 2,3,5,6-tetramethylnitrosobenzene, 3,5-di-bromo-4-nitrosobenzenesulfonate (sodium salt), 2,4,6-tri-t-butyl-nitroso-benzene, and 2-methyl-2-nitrosopropane were investigated using in situ EPR technique. Nitrosobenzene is efficiently photoreduced in TiO2 suspensions (toluene/alcohol, 1:1 (v/v)) forming exclusively one stable radical intermediate corresponding to C6H5NOH species. The formation of this radical species is consistent with the proposed photocatalytic reduction mechanism, occurring from the primary generated nitrosobenzene mono-anion by the hydrogen abstraction from surroundings. The origin of hydrogen added to the nitroso group was demonstrated by the photocatalytic experiments using deuterated methanol, where the production of C6H5NOD was established. Additionally, an identical radical C6H5NOH was detected, when nitrobenzene was reduced under analogous experimental conditions. The photoinduced electron transfer from TiO2 to nitroso compounds is accompanied by alcohol oxidation via the photogenerated titanium dioxide valance band holes forming alkoxy and hydroxyalkyl radicals. Production of hydroxyalkyl radicals (CH2OH, CH(OH)CH3, C(OH)(CH3)2) with redox potentials suitable for a direct electron transfer to nitroso compounds represents an alternative reaction pathway for their reduction. On the other hand, the investigated nitroso derivatives are efficient spin-trapping agents, therefore, formation of nitroxyl radical spin adducts was observed in the photocatalytic experiments. The EPR spectra monitored upon irradiation of substituted nitrosobenzene derivatives in alcoholic TiO2 suspensions reveal the correlation between nitrosobenzene derivative first step reduction potentials and yield of radical species produced.